Oh it has a massive purpose. What happens when Gil can only come into the game rather than out? There will be massive inflation (costs go way up) and that Gil you earn will be useless. That's what would happen if there was no Gil sinks. Square messed up bad at the start of this game. I was level 10 and I had already made my first million from doing Guild Leves and vendoring. I'm one person so imagine how much went in for all those people who are extremists. I have a good portion of cash now that i'm just sitting on because I have no purpose for it but I earned that through mining.
They need to make sure Gil retains a value. I don't want to be paying 200k+ for a level 5 item.